Respirator masks equipped with detachable filters have been used for decades to protect the respiratory system of individuals from noxious gases, vapors, fumes and particulates. The detachable filters are connected to the inlet valves of the respirator mask through which ambient air is drawn as the user inhales. As such, the air is filtered before it is inhaled.
Many respirator filter cartridges are designed as short cylindrical containers having a rigid metal or plastic shell with a filter material such as activated carbon housed inside the container. An example of such filter cartridges is shown and described in U.S. Pat. No. 5,714,126. Due to their rigid shell design, such filter cartridges are inflexible, and if they are thick enough, they sometimes make it difficult for the user to wear certain eye and face protection such as a welding hood. Additionally, sometimes such filter cartridges are thick enough to interfere with a portion of the users vision. In an attempt to overcome these limitations in certain applications where only a particulate filter is required, flexible particulate filters have been designed that easily fit under certain eye and face protection such as a faceshield.
U.S. Pat. No. Re. 35,062 shows a flexible particulate filter having front and back walls of filter material joined to each other along their peripheral edges in a spaced-apart relationship and equipped with a breather tube having an attachment means to secure the filter to a respirator mask. The spaced-apart relationship is maintained by a soft porous layer of specific materials encased within and substantially coextensive with the filter walls. The filter walls must be maintained in a spaced-apart relationship; otherwise the front and back walls would merely collapse together and the filter would not function. While such filters are flexible, their lack of a frame makes them susceptible to being damaged due to soiling and crushing, particularly during attachment and removal. That is, the filter may become dirty or bent such that air cannot be drawn through a portion of the filter. When this happens, the effective surface area of the filter is reduced and the breathing resistance increases. Additionally, the porous layer contributes up to 50% of the total pressure drop of the filter making breathing more difficult.
U.S. Pat. No. 5,732,695 describes a particulate filter which is a hybrid between a flexible filter and a filter cartridge. It has the front and back walls of filter material like a flexible filter but also has a rigid frame having an outer peripheral band connected by a plurality of spacers to an inner breather tube. While this filter has the increased surface area of a flexible filter, the rigid frame prevents flexibility.
The xe2x80x9c75 FFP 100xe2x80x9d particulate filter made by North Safety Products includes a flexible frame that may be described as having a hub and spokes. The hub of the frame forms a centrally located breather tube which attaches the filter to a respirator mask. The spokes of the frame separate the front and back layers of the filter. Unfortunately, the hub and spoke design results in an interior frame having a sharp point and edges at the end of each spoke. When the user grasps the filter to install or remove it, he or she may grip the filter with sufficient force so as to crush the filter and cause the frame spokes to puncture the filter material thereby damaging the filter. Additionally, because the flexible frame does not extend to the outer edge of the filter material, these filters are also susceptible to bending, folding and crushing.
Accordingly, it would be desirable to have a filter which has the benefits of a flexible particulate filter without the known limitations. This can be accomplished by utilizing a flexible internal frame that extends to the edge of the filter material and is substantially coextensive therewith, thereby providing sufficient rigidity to prevent crushing and damage to the filter during installation while still providing sufficient flexibility to enable the filter to fit under other personal protective equipment.
Generally, the present invention is directed to a flexible respirator filter comprising a filter material forming substantially coextensive front and back portions which are joined to each other along their periphery forming a sealed edge. The filter material preferably comprises a plurality of layers for better durability and spark resistance and can even include a layer of activated carbon. A frame is at least partially encased within the front and back portions of the filter material such that a peripheral edge of the frame remains adjacent to the sealed edge of the filter material. Preferably the frame is made from a flexible material such as plastic. Alternatively, the frame could be made of a rigid material which is thin enough to be flexible. While the frame itself is flexible, having a peripheral edge adjacent to the sealed edge of the filter material provides enough rigidity to prevent crushing during installation or removal while not being so rigid that the filter will not bend to fit under a welding mask or faceshield.
More particularly, the present invention provides a flexible filter comprising a flexible frame that is totally encased between the front and back layers of filter material and which remains substantially coextensive with and immediately adjacent to the sealed peripheral edge of the filter material along substantially its entire length. Thus, the shape of the filter is maintained and crushing does not occur. Preferably the internal frame is generally round and does not have any corners or points so that when the user grasps the filter to install or remove it, the filter remains stiff and undamaged. Attachment is also aided by a plurality of finger recesses preferably formed in one side of the frame.
The filter also includes a connector for attaching the filter to a respirator mask. The connector protrudes through an opening in the back portion of filter material and is sealed thereto. Preferably the connector is formed integrally with the frame. Preferably the filter has a generally circular shape with the connector being slightly off-center. By having the connector located slightly off-center, the filter can be maintained in a swept back position when installed on a respirator mask thereby giving the user greater visibility when looking down. Preferably the finger recesses are formed integrally with the frame and are located on opposite sides of the connector.
The flexible frame creates a plenum in the filter so that air entering the front and back portions of the filter material can easily reach an opening in the connector without any increase in breathing resistance. The frame has a plurality of cross-members, preferably at least one cross member lying within the plane of the frame that acts as a stiffening element and several additional cross-members, which do not lie in the plane of the frame and thereby create the plenum between the front and back portions of filter material which facilitates breathing through the filter.
Other details and advantages of the present invention will become apparent as the following detailed description of the invention proceeds.